The present invention relates to extensometric sensors for measuring stresses acting on a drilling element at the bottom of a well, as well as a device for mounting it on the element, for measuring the tension and the twisting moment applied under bottomhole drilling conditions.
The forces exerted on the drilling equipment at the bottom of a well when progressing into the formation are considerable and involve considerably deformations which may possible damage the measuring apparatus situated close to the end where a tool or probe is situated. It is then necessary to analyze the stresses which are exerted on the equipment so as to derive therefrom the modifications to be made to the drilling conditions.
If the stresses exerted on a tube subjected to the bottomhole drilling conditions are studied, it can be seen that they are essentially of three kinds: flexion with respect to the axis of the drilling element, tension/traction along this axis and a twisting moment. Among these stresses, those due to flexion are appreciably greater than those due to the tension or to the twisting moment.
One of the existing methods of evaluating stresses applied in the drilling field consists in bonding stress gauges at appropriate positions along the drilling element and processing the collected signals so as to evaluate the flexion, traction and twisting stresses by decoupling. It can then be discovered that the flexion stresses are exerted in a direction perpendicular to the twisting stresses.
In order to overcome the flexion stresses, if stress gauges are placed diametrically symmetrically with respect to the axis of the tube, the sum of the measurements of these flexion stresses is canceled out. On the other hand, the sum of the traction stresses measured under the same conditions doubles the sensitivity of the measurement.
Thus, theoretically, by appropriately bonding the extension gauges, complexes decoupling is obtained between the flexion, tension/traction and twisting measurements.
However, the imperfections of the metal and the slight bonding errors lead to the observation of so-called transverse sensitivity effects.
Thus, if the element is subjected to pure flexion, a more or less considerable effect will be observed on the measurement of the tension or of the twisting.
This transverse sensitivity is all the greater the lower the lever of the effect it is desired to measure with respect to that of the disturbing signal. This is the case for a tube subjected to bottomhole drilling conditions.